"Why do atheists have parody religions? Do they have to mock theists beliefs?" This is a common question when someone first learns about the many parody religions out there, but the majority of them actually have an education purpose behind their humor, generally against one or several tactics of apologists. I'll try to cover the most well known ones here and explain why they exist to the best of my ability.
Russell's Teapot - A concept created by Bertrand Russell to demonstrate the concepts that the burden of proof rests on the theists making a claim of god(s) and that such claims are not logical unless they are falsifiable. Russell pointed out that if he claimed a teapot was in orbit somewhere between Earth and Mars, it would be nonsensical for others to believe him if he could provide no evidence. Thus "A-Teapotists" aren't making the claim that there is no teapot, but simply rejecting the claim that there is one until proven otherwise.
Church of the Flying Spaghetti Monster (FSM) - Also known as "Pastafarianism", it is perhaps the most popular parody religion. It is primarily used as a parody of creationist and intelligent design apologetics. In response to the Kansas State Board of Education's 2005 decision to allow intelligent design to be taught as an alternative to evolution, Bobby Henderson wrote a letter professing his belief in a supernatural being who resembled spaghetti and meatballs, for which there was just as much evidence as intelligent design and thus should be offered as an alternative. Atheists often challenge theists to disprove the Flying Spaghetti Monster, because doing so points out how flawed their own arguments for god(s) can be.
Invisible Pink Unicorn - Another modern take on Russell's Teapot, the Invisible Pink Unicorn is used to point out that supernatural beliefs are arbitrary, generally by replacing the word "god" with "invisible pink unicorn". It illustrates the nature of unfalsifiable claims a bit better than Russell's Teapot because the unicorn is invisible and thus can't be seen. And if scientific demands are made, she can be magical, always shifting the goal post out of reach. It also points out the absurdity of faith. "How do you know it's pink if it's invisible?" "I just have faith."
The Church of the Latter-Day Dude - Also called "Dudeism", it is a parody of eastern religion, primarily Taoism, based around the Coen Brothers' film The Big Lebowski. It liberally uses quotes and iconography from the cult film and has it's own "holy books", including The Dudespaper, The Tao Dude Ching, The Tao of The Dude, and The Abide Guide. All are parodies of popular theist publications, such as The Watchtower for the Jehovah's Witnesses.
The Church of Google - Makes the claim that, while the Google search engine is not supernatural, it is the closest mankind has come to observing an all-knowing being. Their website even has a list of nine "proofs" that Google is god, similar to other apologists. Among these proofs are that Google is infinite as it is exponentially expanding and that it answers prayers so long as they're done through the search bar.
Last Thursdayism - A joke version of omphalism (the claim that the universe only appears billions of years old because a god made it appear that way to humans) that claims the universe was destroyed last Wednesday, but we're here so the universe must have been recreated last Thursday in order to appear as though it were never destroyed, complete with lifetime memories, geological formations and stars. There are also "heretic" versions, including Last Wednesdayism and Next Thursdayism.
The Great Pumpkin - Created by Charles Schulz in his comic strip Peanuts. It is seen as a Halloween parallel or parody of Santa Claus, believed in only by the character Linus, who refuses to reject his belief despite overwhelming evidence against it. This illustrates, perhaps accidentally (Schulz was a Christian) the absurdity of supernatural claims. For many atheists, this was an early stepping stone to losing their faith and thus they thank the Great Pumpkin for their atheism.
Festivus - "A Festivus for the rest of us!" A parody holiday featured on Seinfeld (and based on real traditions followed by one of the writer's family) which has since been embraced by many as an alternative for Christmas in order to escape its commercialism. The holiday has a "Festivus Pole" in place of a Christmas Tree and has a Festivus dinner immediately followed by the "Airing of Grievances" in which the family members lash out at each other verbally for all their shortcomings. Other traditions include the "Feats of Strength", a wrestling match between the head of the household against another person of their choosing, and proclaiming mundane and easily explainable coincidences to be "Festivus Miracles!"
That's a lot of religions! And I know I've left out plenty, but those are the most popular ones most people are likely to come across. I hope I've illustrated that these religions are mostly used for humor between other atheists and as instructional tools to help theists understand how certain claims appear to those who don't believe in them.
While Secularists aren't as steeped in symbolism and iconography as many religions are, we do still have symbols and concepts that we use to quickly express thoughts and ideas, like all other people and cultures. Here's a few of the most common.
Darwin Fish - A parody of the ichthys symbol (Jesus Fish) seen on many car bumpers that appears as a fish with legs with "Darwin" in the middle , it has since spawned countless variations, including Star Trek fish, (as seen on my boyfriend's truck) FSM fish, Cthulu fish, and many more. This has resulted in a "fish war" in which Christians show an ichthys eating Darwin fish and vice versa. They are primarily used by Atheists as a humorous way of proclaiming their heathenism.
Happy Human - Symbol for Secular Humanism, a moral philosophy held by many atheists (myself included) which is explained by the International Humanist and Ethical Union as "a democratic and ethical life stance, which affirms that human beings have the right and responsibility to give meaning and shape to their own lives. It stands for the building of a more humane society through an ethic based on human and other natural values in the spirit of reason and free inquiry through human capabilities. It is not theistic, and it does not accept supernatural views of reality."
Scarlet Letter - Based on the same-titled Hawthorne book, it has since been adopted as a symbol for atheists and agnostics, which is seen as much worse than adultery by many religious people. It is often seen as a lapel pin worn as a subtle way of identifying other atheists as the symbol is not commonly known outside the secular community.
American Atheists' "A" - Symbol used by American Atheists, a non-profit organization dedicated to protecting the constitutional rights of atheists in the United States and defending the separation of church and state. One of the first and largest groups of its kind, the symbol is often used outside of the organization, including as a grave marker for atheist soldiers.
Cross Buster - Used by the popular atheist punk band Bad Religion, it has since been adopted by many in the atheist community, particularly those with an anti-theist stance toward religion. There are many variations for different religions, including one used by my podcast, Secular Shethinkers.
Eating Babies - A common joke used in the Secular community based on the outrageous claims made against us by religious fundamentalists. (See also, "Worship Satan", "Angry at God", "No moral compass", and "Love to sin") As a result, it's not entirely uncommon to see a baby-shaped cake or other types of food at a secular gathering.
Well, it's not everything but it's a pretty good start. I hope you enjoyed it and learned something new, primarily that atheists are snarky bastards who really like sarcasm. Don't forget to check out Part 1 on Queer Symbols if you missed it and let me know if I forgot anything important. Join me next time as I talk about the misperceptions of feminism.